Maybe in a perfect world... Certainly, there's a reasonable chance that UTF-16(BE|LE) really will start with the "byte-order-mark" (BOM, \x{FEFF}), but oh, so many people are not so reasonable. I've seen UTF16 files created by Microsoft tools that were little-endian (of course) but had no BOM.

As for utf8 files, um, where did this information about "\xEF\xBB" come from? I've never seen a file that starts like that (and I would have thought that any proper utf8 mechanism would barf given this sort of byte sequence -- an initial "\xEF" would dictate the start of a 3-byte character, but you don't indicate a third byte). If you mean \x{EFBB} (expressed as UTF-16), this would be three octets when converted to utf8: \xEE\xBE\xBB).

Regarding the notion of ASCII data, many people don't realize that ASCII files are simply a proper subset of utf8 files -- this was, I think, one of the design goals for utf8. (This adds to my doubts about "\xEF\xBB": this sequence isn't supposed to be in an ASCII file, yet an ASCII file is supposed to work as a utf8 file.)

I think the question, though vaguely stated, may have been more concerned with distinguishing, say, the different flavors of ISO-8859 (which is impractical without knowledge of the language being used in the text, or at least some well-trained n-gram models for various languages), or CP12* vs. Mac* vs. 8859-* vs. euc-*, etc, etc (somewhat less speculative, but still not always simple or deterministic without modeling).

Perhaps more clarification is needed about the scope of the question. ("raw" text files? HTML/XML files? files that are simply "some form of unicode, but I don't know which"?)

When putting a smiley right before a closing parenthesis, do you:

Use two parentheses: (Like this: :) )
Use one parenthesis: (Like this: :)
Reverse direction of the smiley: (Like this: (: )
Use angle/square brackets instead of parentheses
Use C-style commenting to set the smiley off from the closing parenthesis
Make the smiley a dunce: (:>
I disapprove of emoticons
Other